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Effect of antioxidants on four physical properties of polypropylene (PP)

August 25,2022

Antioxidant is added to plastic materials, effectively inhibit or reduce the rate of thermal oxidation reaction of plastic macromolecules, significantly improve the heat resistance of plastic materials, delay the degradation of plastic materials, the aging process, extend the service life of plastic products, plastic products to improve the value of plastic additives.

Antioxidants are the most widely used additives in plastics. One of the most widely used content refers to the application of antioxidants in all different stages of plastics polymerization and synthesis, granulation, storage, processing, and use. The second most widely used content, refers to the world today has appeared in a variety of different molecular structure of the plastic material types, such as polyethylene, polypropylene, styrene polymers, engineering plastics, modified plastics and other materials, the application of antioxidants in the most types of plastic materials.Our products theplastic pipe extrusion lineis suitable for the high-speed extrusion of HDPE, PP and other polyolefin pipes.

Commonly used plastic antioxidants are generally divided into four categories according to the molecular structure and mechanism of action: hindered phenols, phosphite, thio and compound.

Factors affecting antioxidants on polymeric materials
The suitability of the choice of antioxidant system in polymers needs to be evaluated and verified, and since it is to examine its application performance, it is necessary to examine the changes in performance related to its addition to the polymer. For polymeric polypropylene materials, due to their own characteristics, the following properties are mainly examined.

01 Melt index or viscosity
Melt index is an important indicator of the processing performance of polypropylene resin, and the melt index method is usually used to evaluate the processing stability of antioxidant-protected polymeric materials. The greater the change in melt index before and after processing, the worse the processing stability of the material. Therefore, the change in melt index of the antioxidant stabilizer system after several extrusions is examined.

02 Yellow Index
Polypropylene in the process, prone to thermal aging phenomenon, the addition of the main antioxidant inhibition of polypropylene degradation at the same time the formation of quinone chromophores, so that the product color yellowing. Therefore, the polypropylene stabilized system with the addition of antioxidants, after several extrusions with the change in yellow index to evaluate the impact of antioxidants on polypropylene processability.

03 Oxidation induction period
The oxidation induction period is the time for the polymer to react with oxygen in chain auto-oxidation under certain conditions and indicates how easy it is for polypropylene to react with oxygen at a certain temperature (200°C). The larger the oxidation induction period, the more effective the system is in preventing the formation of free radicals at high temperatures. Once free radicals are generated in polypropylene, oxygen is rapidly absorbed and a chain auto-oxidation reaction occurs. Four antioxidant stabilization systems were tested on DSC with polypropylene powder after the first extrusion of the pellets for the oxidation induction period, and the results are shown in Table 3.

04 Long-term heat and oxygen resistant aging
Long-term heat and oxygen resistance aging function is the basic function of plastic material anti-aging or weathering, anti-UV or anti-light aging function is an improved function built on top of the basic function. For modified plastic products used outdoors, the combination of suitable antioxidant system and appropriate addition amount in the light stabilization system can moderately improve the long-term light stabilization effect or effect of the products.

The above experimental data show that different antioxidant systems have different effects on the melt index or viscosity, yellow index, oxidation induction period, and long-term thermal oxygen aging of the material. Polymers choose what kind of antioxidant system, in addition to the conventional varieties, different product requirements, personnel habits, sources of channels, etc. will affect people’s choice, but choose what the most appropriate antioxidant, people need to continue to experiment and production practice, but also rely on scientific and technical personnel to develop new varieties of antioxidants.

As for the technical staff in the polymer industry, when conducting formulation research, they need to select suitable antioxidants and antioxidant combinations through a large number of experiments to develop suitable materials

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